1. Field of the Invention
The present invention relates to an image reading apparatus which illuminates an image recorded on film or the like via an illumination system, and uses a line sensor or the like to convert the illuminated image to electric signals which are output by the apparatus.
2. Description of the Related Art
Conventional image reading apparatus use a line sensor having a charge-coupled device (CCD) to convert an image recorded on film or the like to electric signals which are output by the apparatus, and the image reading apparatus of the aforesaid type typically have an illumination optical system for image illumination, as shown in FIG. 1. The illumination optical system of the aforesaid type is constructed so as to focus via the condensers 3 and 4 the light emitted from the xenon lamp 1 having a reflective hood 2, and project said focused light on a film surface F supported by the film holder 5. The aforesaid projected light has a two-dimensional spread. Item 6 is a projection lens and item 7 is a line sensor.
The optical system of the aforesaid type has a simple construction, but is disadvantageous in that the device is large in size. Furthermore, when the aforesaid optical system uses a line sensor as the image reading means, excessive power is unnecessarily consumed and illumination efficiency is impaired because the area outside the area being read by the line sensor is also illuminated.
Therefore, illumination optical systems having a plurality of light-emitting diodes arrayed along the line sensor, and illumination optical systems which conduct the light projected from a light source such as a xenon lamp and the like to the vicinity of a line sensor via optical fiber and the like have been proposed.
The aforesaid light-emitting diode illumination system increases construction costs because an array of a plurality of light-emitting diodes must be provided, and may cause concern of reduced resolution due to the effects of heat produced by the light-emitting diodes of the line sensor. Furthermore, the illumination system using optical fibers avoids the disadvantage of being adversely affected by the heat generated by the line sensor, but does have the disadvantages of large size and higher cost.
A conventional illumination device using a light guide LG is shown in FIGS. 2 and 3. FIG. 2 is a section view showing the light guide LG in the lengthwise direction, and FIG. 3 is a section view of the light guide LG in the Z--Z direction. This illumination device provides halogen lamp 1 having a reflective hood 2 arranged at one end of the cylindrical light guide LG. The light emitted from the halogen lamp 1 is totally reflected within the light guide LG so as to be directed in the lengthwise direction of the light guide LG and is broadened at the light-diffusing portion R, thereby forming a two-dimensional light source. The light from the aforesaid light-diffusing portion R (two-dimensional light source) is released from a slit SL provided in the surface of the light guide LG.